A general excavator, as illustrated in FIG. 1, includes a lower driving structure 1; an upper swing structure 2 mounted on the lower driving structure 1 to be swiveled; a cap 3 and an engine room 4 mounted on the upper swing structure 2; a working device 11 including a boom 6 fixed to the upper swing structure 2 and driven by a boom cylinder 5, an arm 8 driven by an arm cylinder 7, and a bucket 10 driven by a bucket cylinder 9; and a counter weight 12 mounted on the upper swing structure 2.
A conventional hydraulic system for construction equipment, as illustrated in FIG. 2, includes first and second variable displacement hydraulic pumps 50 and 51 connected to an engine (not illustrated); a first traveling control valve 54 installed in a center bypass path 52 of a first hydraulic pump 50 to control the start, stop, and direction change of a left traveling motor 53; first control valves 58 and 59 installed in the center bypass path 52 on a downstream side of the first traveling control valve 54 and connected together through a parallel line 55 to control hydraulic fluid being supplied to a swing motor 56 and an arm cylinder 57; a second traveling control valve 62 installed in a center bypass path 60 of a second hydraulic pump 51 to control the start, stop, and direction change of a right traveling motor 61; and second control valves 66 and 67 installed in the center bypass path 60 on a downstream side of the second traveling control valve 62 and connected together through a parallel line 63 to control hydraulic fluid being supplied to working devices such as a boom cylinder 64 and a bucket cylinder 65.
If the first traveling control valve 54 is shifted during an independent traveling of an excavator, the left traveling motor 53 is driven by the hydraulic fluid being supplied from the first hydraulic pump 50, while if the second traveling control valve 62 is shifted, the right traveling motor 61 is driven by the hydraulic fluid being supplied from the second hydraulic pump 51. Accordingly, the excavator can travel smoothly.
When a combined work is performed through simultaneous driving of a traveling apparatus and a working device such as a boom, a part of the hydraulic fluid discharged from the first hydraulic pump 50 is supplied to the left traveling motor 53 through the first traveling control valve 54, and simultaneously, another part of the hydraulic fluid from the first hydraulic pump 50 is supplied to the boom cylinder 64 via a control valve 68 provided on the parallel line 55.
In addition, a part of the hydraulic fluid discharged from the second hydraulic pump 51 is supplied to the right traveling motor through the second traveling control valve 62, and simultaneously, another part of the hydraulic fluid from the second hydraulic pump 51 is supplied to the boom cylinder 64 via the second control valve 66 provided on the parallel line 63.
That is, in the excavator to which a hydraulic system using two hydraulic pumps having the same capacity is applied, the left traveling motor 53 and the working device (e.g. the arm cylinder 56 and so on) are driven by the hydraulic fluid discharged from the first hydraulic pump 50, and the right traveling motor 61 and the working device (e.g. the boom cylinder 64 and so on) are driven by the hydraulic fluid discharged from the second hydraulic pump 51, so that the excavator can travel straight during the combined work for simultaneously driving the traveling apparatus and the working device.
On the other hand, in the case of performing a work that produces a great load according to work conditions, a large-scale excavator may be used. In this case, hydraulic pumps, control valves, and actuators of desired capacities are mounted on the excavator. However, if a large-capacity hydraulic pump cannot be used in the excavator (due to its expensiveness and so on), a third hydraulic pump may be additionally installed in the excavator instead.
Another conventional hydraulic system for construction equipment, as illustrated in FIG. 3, includes first and second variable displacement hydraulic pumps 50 and 51 connected to an engine (not illustrated) and so on; a first traveling control valve 54 installed in a center bypass path 52 of a first hydraulic pump 50 to control the start, stop, and direction change of a left traveling motor 53; first control valves 58 and 59 installed in the center bypass path 52 on a downstream side of the first traveling control valve 54 and connected together through a parallel line 55 to control hydraulic fluid being supplied to a swing motor 56 and an arm cylinder 57; a second traveling control valve 62 installed in a center bypass path 60 of a second hydraulic pump 51 to control the start, stop, and direction change of a right traveling motor 61; second control valves 66 and 67 installed in the center bypass path 60 on a downstream side of the second traveling control valve 62 and connected together through a parallel line 63 to control hydraulic fluid being supplied to working devices such as a boom cylinder 64 and a bucket cylinder 65; and a third variable displacement hydraulic pump 69 connected to the center bypass path 60 on an upstream side of the second hydraulic pump 51 and the parallel line 63 to increase the supply amount of hydraulic fluid to the working device so that the driving speed of the working device is increased.
In this case, the construction except for the third hydraulic pump 69 for additionally supplying the hydraulic fluid so as to increase the speed of the corresponding actuator of the working device is substantially the same as the construction of the hydraulic system as illustrated in FIG. 2, and thus the detailed description thereof will be omitted. Also, the same drawing reference numerals are used for the same elements across various figures.
In the case of increasing the driving speed of the corresponding working device (e.g. the boom cylinder 64) on the second hydraulic pump side 51 by the hydraulic fluid being supplied from the third hydraulic pump 69, a part of the hydraulic pump discharged from the third hydraulic pump 69 is also supplied to the traveling motor. Accordingly, the supply amount of hydraulic fluid becomes unbalanced due to the difference in load pressure occurring between the working device side and the traveling apparatus side, and thus the straight traveling of the construction equipment cannot be secured.